reasons are:
-high range
-good/best scalings compared to other ad (attack speed, ad/lvl)
-mana less
-PRETTY HIGH DMG ALL GAME STAGES (early/midgame with his spells)
-more durable than other ad
-brings aoe/single stun
-can chase/escape really good
-easy farming, even when having hard time in lane due his q
-he is fun to play
he is really nice. his damage is lower than say a vayne lategame, but he brings lots of utilty and despite people saying his ult doesnt deal dmg when he goes ad, its 630 base dmg to the same target at lvl 16, which is quite nice, especially if you consider it hitting multiple people.
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im not saying ad kennen is best, i kind of agree ap kennen is stronger, but ad kennen isnt bad and is absolutely viable